DK155474B - OPTICAL POLARIZATION PRESERVING FIBER OF SINGLE-WAVE TYPE AND PROCEDURES OF PRODUCTION THEREOF - Google Patents

Description

'DK 155474 B

BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to a single-wave optical polarization-preserving fiber of the type comprising a transparent core surrounded by a layer of transparent coating material having a refractive index lower than the refractive index of the core 5, and wherein the coating layer has an asymmetry that produces double refraction in the core.

In many applications of single-wave optical conductors, e.g. gyroscopes, sensors, etc., it is important that the migrating optical signal maintains the polarization-1 characteristics of the arriving light when external depolarizing disturbances occur. This implies that the conductor must have azimuth asymmetry in the refractive index characteristic.

A slight improvement in the polarization properties of 15 single-wave optical conductors can be achieved by distorting the fiber-chore symmetry as a way of decoupling differently polarized waves. Two such fiber-wave conductors are known from the disclosure of U.S. Patent No. 4,184,859 and from the publication: "Influence of Noncircular Core on the Polarization Performance of Single Mode Fibers", V. Ramaswamy et al., Electronics Letters, Volume 14 , No. 5, pages 143-144, 1978. However, the Ramaswamy publication states that measurements of borosilicate fibers with non-circular cores suggest that the non-circular geometry 25 and the associated impact-induced birefringence are not sufficient in themselves to maintaining polarization in single-wave type fibers.

The invention described in GB Patent No. 2,012,983 is based on the recognition that orthogonally polarized waves are decoupled more efficiently in a waveguide which is designed to intentionally emphasize double refraction induced by external influence. The teaching of this patent is that this is achieved by introducing a geometric or material asymmetry into the subject from which the optical fiber is drawn. The influence indus- (in 2

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double birefringence is introduced by at least partially enclosing the single-wave type conductor with a sheath having a different coefficient of thermal expansion than the waveguide itself and in one direction a thickness different from the thickness in another direction perpendicular to the first mentioned direction. . For example, the starting blank may be a three-layer structure comprising an inner core region surrounded by a cover layer which is in turn surrounded by a jacket having a different coefficient of thermal expansion than the cover layer itself.

Diametrically opposed portions of the outer layer are abraded, and the resulting blank is drawn to a fiber which is generally shaped like a tile, the sheath having different thickness of thickness in two perpendicular directions. A similar result can be obtained by designing the output blank with an inner core region, a coating and two opposed outer shell forming layers disposed along the surface of the output blank. It can be difficult to produce a starting material of this kind, as tension in the outer layer occurs. When grinding the outer layer, or when grooves are cut in this layer, the occurring tension tends to cause the workpiece to break.

Assuming that a fiber can be pulled out of the starting blank, the tension of the exterior layer will be at a relatively large distance from the fiber core, so the effect of the stress on the choir is minimal.

In the embodiment shown in FIG. 10-15 of said GB patent 2,012,983, a relatively thick substrate tube forms the outer portion of the optical fiber. To impart the desired properties to the fiber, either the inner surface or the outer surface of the substrate tube is non-circular. Since at least part of the substrate wall must be relatively thick, the application efficiency is adversely affected. Since the substrate tube also forms the outer compressible layers of the fiber, it may happen that the commercially available tubes cannot be used in the process unless the 3

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happens to possess the desired expansion and / or viscosity properties of the resulting fiber outer layer.

The outer coating layer 60 on a fiber of the kind shown in FIG. 12 of said GB patent 2,012,983, 5 is referred to herein as voltage coatings. It has been found that the voltage σ in the choir of an optical circular symmetric single-wave type fiber is equal to the product f * g, where f is a function of geometric factors and g is a function of glass factors. The function f is defined by the equation; 10

Asc * - s5- (i)

Of which A is the cross-sectional area of the tension coating, and is the total cross-sectional area of the fiber. The function f can therefore have a value such that 0 <f <1. The function g is defined by the equation: _ _ Ε (Δ «) ΔΤ 3" 2 (1-0 '(2) 20 where E is the fiber's effective modulus of elasticity, Δ «* is the difference between the heat expansion coefficient of the voltage coating and the heat expansion coefficient of the remaining part of the fiber, ΔΤ is the difference between the lowest solidification point (so-called set point) of the glass materials of which the fiber consists and the room temperature, and v is Poisson ratio Since the above definition of voltage σ generally also applies to non-symmetrical fibers such as the fibers disclosed in GB Patent No. 2,012,983, it is necessary to increase f as much as possible. to obtain the maximum chord voltage and thus the maximum voltage double breaking. Values of f greater than 0.9 must be obtainable to give maximum values of voltage double breaking. The need to make the function f maximum is shown in Equations-35. ne (7) and (8) of said GB patent No. 2,012,983.

I i 4

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Another well-known design criterion for single-wave optical type conductors is related to the problem of minimizing losses. A common method for providing output blanks for single-wave optical conductors is illustrated in FIG. 11 of GB Patent No. 2,012,983, showing a plurality of vapor-applied layers on the inside of a substrate tube. The purity of the substrate tube is usually not as high as the purity of the vapor applied glass. Therefore, the vapor-applied glass is insulated from the substrate tube by a layer of vapor-applied optical coating glass of sufficient thickness. For a single-wave type fiber having a core of circular or approximately circular cross-section, the radius rg of the optical coating must be at least five times the radius ra 15 of the core. This assessment is based on the results stated in the publication: Electronics Letters, Vol. 13, No. 15, pages 443-445 (1977). For fibers whose cores are of elongate cross-sectional shape, this relation has no particular significance. For such a fiber, the range of 20 optical coatings can best be expressed in relation to its thickness * Since the size of a single-wave type chore is related to the transmission wavelength λ, the thickness of the optical coating can also be expressed in relation to λ.

The aforementioned ratio of coating radius to choreographi implies that the optical coating should have a thickness of at least 20λ. When a single-wave type conductor is designed in accordance with this criterion, the loss associated with the thickness of the coating will be limited to an acceptable low value.

The subsequent analysis of GB patent no.

2,012,983 include with respect to the specific embodiment described with reference to FIG. 10-12 of said patent. The fiber in this embodiment meets the requirement that the ratio Asc / A ^ should be greater than 0.9 except when the substrate tube is completely

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5 filled with inner layers during the process of producing the starting material from which the fiber is drawn. The said exception is obviously an impossibility. Since the substrate tube cannot be completely filled during the process of applying the inner layer, the total thickness of the inner layers will be limited by the inner diameter of the substrate tube. It is well known that the chore diameter of a single-stage single-wave type fiber profile is usually between 3 µm and 10 µm. The outer diameter of the fiber is usually approx. 125 ym.

10 If the starting blank described in GB Patent No. 2,012,983 is provided in a conventional manner such that the ratio A / A A is greater than 0.9, the thickness of the optical coating layer will be less than 20λ at the conventional wavelengths. Therefore, the excess fiber losses due to insufficient optical coating thickness will not be of sufficiently low value for many applications.

The invention therefore aims at an improved single-polar optical single-wave type conductor exhibiting 20 voltage-induced birefringence.

To this end, an optical fiber of the type mentioned initially according to the invention is characterized in that the coating layer comprises a pair of diametrically opposed, longitudinal glass regions, whose heat expansion coefficient is different from the heat expansion coefficient of the glass.

The invention also relates to a single-wave optical polarization-preserving fiber of the type comprising transparent cores surrounded by a layer of transparent coating material with refractive index lower than the refractive index of the choir, and wherein the coating material forms an outer covering of cladding glass surrounding an inner cladding and a heat expansion coefficient different from heat expansion 6

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the coefficient of the inner lining, the outer surface of the outer casing having generally a circular cross-sectional shape, and wherein the lining layer has an asymmetry which produces double breaking in the choir, the fiber being characterized in that the choir has an elongated cross-sectional shape, that the lining layer is an inner elongated lining layer on said surface of said chancel, and said inner liner comprises a very clean glass optical liner surrounded by a layer of lower purity glass.

Said optical fiber can be formed by providing a tubular intermediate comprising an inner layer of chamfer glass surrounded by a first layer of coating glass. The intermediate is collapsed to obtain a flattened starting blank wherein the co-regias is transformed into a monolithic layer of elongate cross-sectional shape. This chimney glass is surrounded by an inner coating layer which now has elongated cross-sectional shape. A layer of particulate glass, often called soot, is applied to the outer surface of the inner coating layer, the coefficient of thermal expansion of the particulate glass being different from the thermal expansion coefficient of the inner coating glass. The resulting blank is heated to transform the particulate glass into an outer coating glass layer, forming a solid glass blank which can be drawn into an optical fiber.

One method of producing the tubular intermediate is that a number of layers are applied by a chemical vapor application technique to the inside of a substrate tube consisting of glass of lower purity than the glass layers deposited therein. The inner layer forms the core, and the next adjacent layer, which is thicker than the core layer, forms the optical coating. This way of making the fiber allows the optical coating to achieve a thickness greater than 20λ at the working wavelength. Thus, the choir is suitably isolated from 7

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the impure substrate tube.

As another option, the tubular intermediate may be provided by a flame oxidation technique. Reactant vapor is fed to a burner where they are oxidized in a flame to form layers of glass particulate material applied to a cylindrical mandrel. The first applied layer forms the core material in the resulting fiber. At least one additional layer is applied to the first layer to form the inner coating. After removal of the mandrel 10, the resulting hollow porous starting material may be reinforced to form a glass tube, which is then heated on two opposite sides, thereby causing the tubes to collapse. As another option, low pressure can be applied to the soot outlet blank opening to cause it to collapse during the reinforcement process.

Both of these methods allow a very thick voltage-coating layer to be obtained so that the ratio Asc / A ^ is greater than 0.9.

The fiber of the latter embodiment 20 can be provided as follows. A first layer of particulate glass is applied to a rotating mandrel. A second layer of refractive index particle glass lower than the first layer is applied to the first layer. A first and a second longitudinal region, respectively, of particulate glass with heat expansion coefficient different from the coefficient of the second layer is provided on diametrically opposed regions of the second layer. This can be achieved by stopping the rotation of the mandrel, moving the applicators along the mandrel, turning the mandrel 180 ° and again moving the applicators along the mandrel. Alternatively, the longitudinal regions can be formed by changing the composition of the reactant materials supplied to the applicants during the rotation of the mandrel so that the desired particulate glass materials are provided and deposited during each step of the mandrel rotation. Then an 8 is applied

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coating of particulate coating material outside of the resulting body. The coefficient of thermal expansion of the coating glass is the same as that of the second glass coating, and the refractive index of the coating glass 5 is equal to or less than the refractive index of the second glass coating. The mandrel is removed and the resulting porous blank is converted into an optical fiber.

An appropriate method of preparing an optical fiber starting blank for use in the manufacture of an optical polarization preserving fiber of the above type, and providing a lining tube, a glass rod is placed centrally in this tube, which is placed in a pair of glass bars on diametrically opposed sides of the first glass rod, and several glass bars are placed at least in some of the spaces between the central rod, the diametrically opposed bars and the tube, according to the invention, characterized in that as a central rod in the glass tube uses a rod consisting of an axially located 20 choir, surrounded by a layer of cladding glass, and for said opposing glass bars, rods of glass having heat expansion coefficient different from the heat expansion coefficient of the cladding layer. The resulting combination can be drawn into fibers. It is possible to increase the birefringence by placing in the tube on opposite sides of the first rod and orthogonally relative to the first pair of glass bars a second pair of glass bars having different physical properties than the first pair of glass bars.

The fiber of the invention may also be produced by a method according to which a gas is passed through a first tube which, when heated, provides glass particles and by moving a heat source along the outside of the first tube, thereby 35 part of the gas is converted into particulate matter 9

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ale, and at least a portion of the particulate material is deposited on the inside of the first tube. The improvement of the invention is to move a pair of tubes within the first tube, while the ends of the pair of tubes located within the first tube are kept away from the heat source and on the upstream side of the heat source where the pair of tubes is. positioned symmetrically with respect to the center of the first tube. The gas is passed through the first pipe and the pipe pair. Through the pipe pair, a second gas is reacted in the hot zone to form an oxide which is combined with the particulate material to form a glass region which has a coefficient of expansion other than the coefficient of the glass particles provided by 15 the first gas. The pair of tubes can be at least partially drawn from the first tube except when the gas passes through it.

The invention will be explained in more detail below with reference to the schematic drawing, in which FIG. 1 is a sectional view through an intermediate product used to form the starting blank with which the fiber of the invention provides FIG. 2 shows an apparatus used to get it in FIG. 1 to collapse, FIG. Figures 3 and 4 are schematic views of an apparatus for producing a composite starting blank having a coating of soot; 5 shows a sectional view through a blank which is obtained by reinforcing it in FIG. 4 illustrates the composite blank shown in FIG. 6 is a section through a single-wave, single-polarization fiber made by means of the one shown in FIG. 5; FIG. 7 illustrates a flame hydrolysis process for producing a blank comprising a core and a surface.

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drawing on the choir, fig. 8 shows that in FIG. 7, after the mandrel has been removed, fig. 9 shows the reinforced blank; FIG. 10 is a schematic representation of a reinforcing furnace which can be used to reinforce the embodiment of FIG. 8, FIG. 11 is a cross-section through an optical fiber made in accordance with the invention; FIG. Figures 12 and 13 are sectional views of further embodiments of the invention; 14 shows an apparatus for providing one of the embodiments of the invention; FIG. 15 is a sectional view through a finished soot-15 starting blank manufactured by means of the one shown in FIG. 14; FIG. 16 is a sectional view through a fiber made from the one shown in FIG. 5, FIG. 17 is a sectional view through a portion of a modified soot starting blank; FIG. 18 is a sectional view through a workpiece of the type comprising bars in the interior of a pipe; FIG. 19 shows another apparatus which can be used to produce a fiber in accordance with the invention; FIG. 20 is a sectional view of a blank made with the one shown in FIG. 19, and FIG. 21 is a section through a fiber which can be made from the one shown in FIG. 17 or from the 30 shown in FIG. 20.

FIG. 1-10 relates to an embodiment in which a single-wave type fiber has an elongated core surrounded by an elongated inner lining layer and an outer layer cover glass which provides a stress effect and having a circular outer surface. FIG. 1 shows an intermediate 10 which

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11 is provided by the prior art chemical vapor application technique, wherein one or more layers of glass are provided on the inside of a substrate tube which will later form at least a portion of the cover material.

The reactant vapor, together with an oxidizing medium, passes through the hollow cylindrical substrate tube 12. The substrate and vapor mixture in the tube are heated by a source which is displaced along the tube, thereby providing a hot zone within the substrate tube 12. A suspension 10 of particulate material which is provided within the hot zone circulates in the direction of flow and a portion of the material is deposited on the inside of the tube 12 where it is melted to form a continuous glassy coating. The process parameters such as temperature, flow rate, reactants, etc. are disclosed in U.S. Pat.

4,217,027 and in the publications: J. B. MacChesney et al., Proceedings of the IEEE, 1280 (1974) and W. G. French et al., Applied Optics, 15 (1976). Reference is also made to "Vapor Deposition" published by C. F. Powell et al., John Wiley & 20 Sons, Inc. (1966).

Sometimes, initially, a thin barrier layer 14 of pure silica or of silica doped with an oxide such as B2O2 will be deposited on the inside of the tube 12, which usually consists of silica or 25 high silica glass, the purity of the glass being less than the purity of the vapor-applied layers provided therein.

The barrier layer prevents migration of hydroxyl ions and other light absorbing impurities from the tube 12 to the optical coating layer 16. In order to reduce the light transmission losses due to impurities in the substrate tube, the layer 16 is provided a sufficient thickness to allow the thickness of the optical coating layers in the resulting fiber 35 are greater than 20λ. Since the barrier layer 14 is optional, 12 is

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not shown in FIG. 3-6. The optical coating layer is a relatively thick layer of glass with a relatively low refractive index. Conventionally, it may consist of pure silica or silica doped with a small amount of oxide, in order to reduce the process temperatures. The addition of a small amount of Ti3 · applied silica coating layers is recommended in the publication: S. Sentsui et al., "Low Loss Monomode Fibers With P20jj-SiC> 2 Cladding in the Wavelength Region 1.2-1.6 10 µm", 5th European Conference on Optical Communications, Amsterdam, September 1979. The use of P2 ° 5 together with one B2 O3 or F in the applied coating of silica is mentioned in the publication: BJ Ainslie et al., "Preparation of Long Length of Ultra Low-Loss Single- Mo-15 de Fiber ", Electronics Letters, July 5, 1979, Volume 15, No. 14, pages 411-413. The use of such doping materials has led to an application temperature of approx. 1500 ° C, which is approx. 200 ° C lower than the temperature required to apply a coating of pure molten silicon oxide. After applying the optical coating layer 16, a thin layer 18 of core material is applied to the inside of this coating layer. The core layer 18 consists of very pure glass with a refractive index greater than the index of the coating layer 16. The layer 18 can conventionally consist of silica doped with low-loss oxide in order to increase the refractive index. Many dopant materials have been used to prepare the core in single-wave optical fibers, but usually preferred to use GeC 2 · Single-wave type fibers 30 with losses less than 1 dB / km in the infrared range have a core consisting of SiO 2 doped with Ge02 cf. the above publications by Sentsui and Ainslie. The resulting intermediate 10 has an opening 20.

For use at wavelengths in the range of 1.1 to 1.8 33 microns, a preferred intermediate could be prepared as an alternative.

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U.S. Patent No. 4,154,491. This patent discloses the provision of a coating layer of P20 doped SiO2 on the inside of a borosilicate substrate tube with subsequent application of a thin layer 5 of pure SiC> 2 to prevent P20 from diffusing into the GeO2 ~ cores applied to the inside of the layer of pure SiO ?.

With respect to the present invention, it is merely required that the intermediate product 10 has an inner layer of chore-10 glass surrounded by a layer of optical coating layer having a lower refractive index. The choir 18 can e.g. is applied directly to the inside of the tube 12 if the tube 12 consists of very clean glass. The term "inner lining layer" as used herein is used to denote the tube 12 and any other layer or layers of glass surrounding the core layer 18 of the intermediate 10.

The present invention has the advantage that glass tubes available commercially can be used for the substrate tube 12. The cross-sectional area of the coating layer 16 can be made much larger, for example, twice as large as the cross-sectional area of the substrate tube 12, so that it is rather the physical properties of the applied layer 16 than of the tube 12 that are overweight in the determination of properties such as the coefficient of thermal expansion of the interior. clothing. In such a situation, the cross-sectional area of the substrate tube is so small relative to the entire cross-sectional area of the resulting fiber that in practice its physical properties have no significance.

The intermediate 10 can collapse 30 in the manner illustrated in FIG. 2. Burners 22 and 24 provide flames 26 and 28, respectively, on the two opposite sides of the intermediate 10. During this process, the intermediate 10 may be disposed in the holder (not shown in simple so-called glass lathe) in which it was placed during the provision of layers 14 and 14

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16. During the collapse or collapse process illustrated in FIG. 2, the rotation of the holder is interrupted so that only opposing sides of the product 10 are heated. During the collapse process 5, a controlled internal pressure is preferably maintained, as disclosed in U.S. Patent No. 4,154,491. During this step, the heat source must cover a sufficiently wide axial region of the intermediate 10 for it to collapse. Alternatively, a single heat source 10 may be used in the manner disclosed in U.S. Patent No. 4,184,859, whereby it is first one side and then the other side collapsing.

A complete collapse of the intermediate product 10 results in the provision of a blank 30, in which opposing sides of the core layer 16 now form a core portion 32 having elongate cross-sectional shape. In this way, a choir with high aspect ratio can be obtained. The choir is surrounded by an inner lining 34 and a substrate 36, both of which are elongated.

The blank 30 is then provided with a casing whose outer surface is generally circular. The surface of the workpiece 30 is processed in a conventional manner prior to application of the outer coating. The surface of the blank 30 is kept clean after the heat treatment which has led to the collapse of the intermediate 10, the blank 30 being placed in a clean, sealed bag of e.g. polyethylene. If the workpiece 30 is handled or soiled, more cleaning operations are required. It is washed in deionized water and then in a bath of isopropyl alcohol. Then clear etching is performed in HF to remove a layer of glass of a few μ, approx. 1% of the weight of the item. The blank 30 is then purified in deionized water and degreased in isopropyl alcohol and then placed in a clean bag of polyethylene. Soot of desired glass nature is applied to blank 30 by a conventional flame hydrolysis process of the same kind as described 15

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U.S. Patent Nos. 3,737,292 and 4,165,223. FIG. 3 and 4 show an apparatus of the kind now used in the conventional manner in the manufacture of low-loss optical fibers. A flame hydrolysis burner 40 which is supplied with fuel, 5 reactant gas and oxygen or air gives off a flame 38 containing glass soot. Burners of the kind disclosed in U.S. Patents Nos. 3,565,345, 3,565,346, 3,609,829 and 3,698,936 may be used. The necessary liquid constituents for forming glass soot can be supplied to the burner by any of the systems known per se for reactant agents. In this regard, reference is made to U.S. Patents Nos. 3,826,560, 4,148,621 and 4,373,305. An excess of oxygen is added to the burner so that the reactant vapors are oxidized 15 within the flame 38 to form the glass soot directed to the blank 30.

According to a technique for producing the outer covering layer, longitudinal strips 44 and 46 are first deposited on the flat side walls of the blank 30 to accelerate the provision of the circular outer covering. While the holder is stationary, the burner 40 is guided back and forth sufficiently to form a soot layer 44. The blank 30 is rotated 180 ° and another soot layer 46 is applied to the opposite side as shown in FIG. 4. The topsoil 48 of garment sod is then applied as the blank 30 rotates while the burner 40 moves along the blank.

The application of the strips 44 and 46 can be avoided without having too much influence on the geometric shape of the resulting fiber. If the coating layer 48 is directly applied to the blank 30, the soot flow from the burner will deposit a greater amount of soot when the planar sidewalls of the blank 30 face the burner than when the rounded portions of the blank face the burner, since the soot deposition efficiency depends on the size. of the subject as a goal. This causes a more and more circular shape as the layer 48 becomes thicker. In the main, circular cross-sectional shape is obtained, «16

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when the outer diameter of the layer 48 is sufficient, relative to the size of the core, for the resulting fiber to act as single-wave type fiber. The layer 48 must be of sufficient thickness that the Agc / Af ratio of the resulting fiber is greater than 0.9.

The coating layer obtained by flame hydrolysis is porous and must be heated to melt or reinforce to form a glass layer without interfaces between the particles. This consolidation is preferably achieved by treating the composite blank 50 in an oven in the manner set forth in DS Patent No. 3,933,454.

The resulting glass blank 56 may not be circular unless layers 44 and 46 have been applied, or if applied in such a way that they do not offset the lack of circularity of blank 30 initially. The extent to which the outer surface of the workpiece 56 differs from the circular shape decreases as the outer covering 48 becomes thicker.

The FIG. 5 is placed in a furnace 20 where at least one end of the blank is heated to a sufficiently high temperature so that a fiber 70 as shown in FIG. 6. As the fiber 70 is pulled, the surface tension tends to make the outer surface more round.

Figures 7-10 show other methods of producing the intermediate. As shown in FIG. 7, a cylindrical mandrel 85 is applied to a first glass soot coating 84 by a conventional flame hydrolysis process of e.g. the above species. A flame hydrolysis burner 87 emits a glass soot containing flame 86 which strikes the mandrel 85. After a mandrel glass coating 84 is provided on the mandrel 85, the composition of the reactant gas introduced to the burner 87 is changed and another coating 88 of inner liner glass, the outer surface 35 is applied to the first liner 84. The liner 84 has a greater refractive index than the liner 88. The physical properties of the liner 88, e.g. its heat expansion coefficient, is chosen such that the inner lining 17

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in the resulting optical fiber, voltage is imparted to the required extent.

When the coating 88 has reached the desired thickness, the mandrel is removed so that as shown in FIG. 8, 5 obtains a porous blank 90 with aperture 89. The resulting hollow soot can then be consolidated in the same manner as described above to obtain a hollow intermediate 10 'as shown in FIG. 9. The intermediate 101 can then be collapsed 10 as described with reference to FIG. 2, and it can be further treated as described with reference to FIG. 3-6 for forming an optical polarization preserving single-wave type fiber.

The FIG. 8 can also be consolidated in a single processing step in the manner shown in FIG. 10, in order to obtain a high aspect ratio item. After removing mandrel 85 from the soot blank, a tube 91 is inserted at one end of the blank. Then, the blank is suspended on a tubular holder 92 20 by means of two platinum wires, of which only one thread is shown at 93. The end of the gas supply tube 91 protrudes from the tubular holder 92 and into the adjacent end of the blank 90. The blank is consolidated by it is gradually introduced into a furnace 94 in the direction indicated by arrow 97. The subject should be gradually consolidated so that it is only its bottom end that is consolidated, after which the consolidation process proceeds and reaches the end that is located. at the tubular holder 92.

During the consolidation process, flux gas such as helium, oxygen, argon, neon or mixtures thereof may be added to the furnace as indicated by arrows 95. Before the workpiece 90 begins to consolidate, dry gas can be supplied to the aperture 89 in the manner set forth in U.S. Pat.

4125388. As the tip of the workpiece begins to consolidate, the pressure in the aperture 89 is reduced relative to the print outside the workpiece. This can be achieved by connecting a vacuum source to the gas supply pipe 91 through a conduit 96. While the blank 90 is fed into 18 "

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the oven in the direction indicated by arrow 97, the low pressure in opening 89 causes the blank to collapse first in the region at the initially consolidated tip of the blank. As the workpiece is consolidated, the remaining portion of the aperture eventually collapses. In this way, in a single consolidation step, a porous soot 90 formed with the aperture 89 can be consolidated, at the same time as the aperture collapses to form a 10-type blank shown at 30 in FIG. Third

Referring again to FIG. 4-6. The soot layer 48, as well as the strips 44 and 46, if applied, are such that the coefficient of thermal expansion of the resulting coating layer 74 is much greater or much less than the coefficient of thermal expansion of the remaining portion of the fiber 70. It is known that the portion 72 comprising the core 80, the substrate tube 82 and any layers forming the inner liner 78 will be under tension if the coefficient of thermal expansion 20 of the outer layer 74, i.e. the so-called "stress layer" is lower than the effective heat expansion coefficient of the portion 72. Conversely, the portion 72 will be under pressure if its effective heat expansion coefficient is less than the coefficient of voltage coating 74, cf. the publication: ST Gulati and Η. E. Ha-gy, American Ceramic Society 61 260 (1978). In addition, in chore 80 there will be a voltage distribution according to the relation σχ> σ ^, where σχ and the voltages in the chorus are parallel to and perpendicular to the major axis in the cross-section of the chorus. This voltage difference grows with increasing chore height / width ratio. It is this voltage difference that causes the desired birefringence.

A voltage of 20-40 kpsi is required in the choir to induce the desired birefringence. With the height-to-width ratios obtainable by the methods described above, the difference between the thermal expansion coefficients of the inner lining and the outer tension lining should be greater than 1 · 10 ° / ° C. It is then stated

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19 theoretical examples in which the glass compositions for the different parts of the fiber are selected so that the fiber core is under compressive and tensile stress, respectively.

A fiber is provided as shown in FIG. 6 with 5 are the glass compositions given in Table 1. The table also indicates the coefficient of heat expansion for each composition.

Table 1 10

Composition (weight percent)

Heat expansion coefficient Ge02 SiC> 2 cient (x 10 V ° C)

Choir 15 85 13 15 Inner casing 100 5 Pipes 100 5

Outer clothing 30 70 23

The fiber of Table 1 has a core in which there is 20 compressive tension and an outer covering where there is tensile tension. However, although there is an appropriate tension in the choir, the fiber may have insufficient strength. Such a fiber should be reinforced by applying a further coating layer of e.g. SiO2 with low coefficient of expansion.

A fiber of the one shown in FIG. 6 can be made with the materials listed in Table 2 in order to impart tensile stress to the choir.

Table 2

Composition (weight percent)

Varmeudv.-

Ge02 P205 S ± © 2 Ti02 coeff. (x 10_7 / ° C) 35 Kores 15 1.5 83.5 15

Inner clothing 1.5 98.5 6

Outer clothing 93 7 0 20

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A fiber of this type in which the choir is under tensile stress is preferable, as there will be compressive stress in the outer lining, which gives the fiber a greater strength.

FIG. 11-21 relate to a further embodiment of the invention in which the fiber core is imparted a voltage-induced birefringence by providing longitudinal regions of glass in the cladding on opposite sides of the choir which have a different coefficient of thermal expansion than the remainder of the cladding. . FIG.

11 shows a cross section through a single polarization optical fiber comprising a groove 110 surrounded by an inner lining region 111. Diametrically opposed to the groove 110, there are two narrow longitudinal regions 112 consisting of a material having a different coefficient of thermal expansion than the coefficient. for the material 111. While the regions 112 are shown with a somewhat random cross-sectional shape in FIG. 11, the following methods are described which enable various areas to be given various regions. When a fiber is pulled, the longitudinal regions 112 and the perpendicular regions thereof are shrunk differently, so that the regions 112 are subjected to tensile or compressive stresses depending on the value of their heat expansion coefficient relative to the coefficient of the garment.

A voltage-induced birefringence thus induced in the fiber reduces the coupling between two orthogonally polarized main wave types. Around the areas 112 there is a covering area 113, the refractive index of which is preferably equal to or less than the index of the inner covering area 111. The area 113 can e.g. consist of any of the above-mentioned materials used to form the region 111.

35 As shown in FIG. 11, the outer surface of the region 113 may have a circular shape, but it may also have flat areas as described in the above-mentioned GB patent 2,012,983 for centering a fiber relative to a source.

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21 for polarized light or in relation to another fiber with which it is to be connected. If the outer surface of the liner 113 is of generally circular shape, for centering purposes, means such as a longitudinal depression 114 may be provided. If it is preferred to have a fiber having a generally circular outer shape, the entrance end of the fiber may be oriented correctly when the fiber is mounted in a facility. The input end is connected to a source of polarized light and an analyzer is coupled to the output end of the fiber 10. The input end of the fiber is rotated relative to the source until a maximum or minimum is detected in the light emitting from the output end. After detecting a maximum or minimum in the output light, the input end of the fiber is fixed relative to the polarized light source 15.

The regions 112 should be positioned as close to the choir 110 as possible without adversely affecting the light transmission characteristics of the fiber. If the regions 112 consist of low-loss material with the same refractive index as the inner lining 111, the minimum radius r 1.5, where ra is the radius of the choir 110. An refractive index of the regions 112 can be fitted to the refractive index of the cladding by using a cladding consisting of SiO 2 and by providing the voltage-inducing regions 112 on the basis of of e.g.

S1O2 doped with one of the following combinations of doping oxides: GeO2 and B203 or P2 05 and B203 or GeC2 / ^ 2 ^ 5 ^ 2 ^ 3 'As an example of a suitable high heat expansion coefficient composition and in the main case the same refractive index as pure SiC 2 can be selected SiC> 2 doped with 12 wt% B303 and 4 wt% P2 ° 5 * To ensure that the resulting fiber exhibits low losses, at least the entire central region, such as the choir and the inner lining is provided by chemical vapor application. If the refractive indices of these two regions are not matched and if the rm is too small, i. less than approx. 1, 5r, the areas 112 cl

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22 could cause light transmission loss due to scattering.

If the regions 112 adversely affect the light transmission characteristics of the fiber, for example, if the regions 112 consist of a material which absorbs too much light at the transmission wavelengths, the inner radius rm of these regions should be at least three times, preferably at least five times the radius of the choir. This is in line with the teachings of the publication: Electronics Letters, Volume 13, No. 15, pages 443-445 (1977). It is clear that the adverse effect of the light absorbing material grows with decreasing distance between this material and the choir. However, the magnitude of the double breaking in the choir will decrease with decreasing inner radius rm to the longitudinal, voltage-inducing regions. The optimum inner radius of the regions 112 depends on the specific type of single-wave type conductor used, since the amount of light traveling outside the chore area in a single-wave type fiber depends on such parameters as the radius and refractive index.

A fiber may contain a second set of diametrically opposed standing longitudinal regions having physical properties different from those of the first set of voltage inducing regions. The FIG. 12, a core 116 comprises an inner cladding region 117 25 and an outer cladding region 118. Two longitudinal regions 119 having a different coefficient of thermal expansion than the cladding regions are diametrically opposed to the chorus 116. In orthogonal position relative to the regions 119 there is another 30 pairs of longitudinal regions 120 consisting of light absorbing glass or glass having a coefficient of thermal expansion different from the coefficient of region 117 in a direction different from the direction in which the coefficient of thermal expansion of region 119 differs from coefficient 35 for the region 117. For example, the thermal expansion coefficient of the regions 120 may be less than the coefficient of the region 117 if the thermal expansion coefficient of the regions 119 is greater than the coefficient.

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23 cient for the area 117.

For example, if the cladding regions consist of pure SiC 2, the regions 119 may consist of S102 doped with and while the regions 120 consist of the SiC4 doped with TiC 2. In that case, there will be tensile stress in the regions 119 and compressive stress in the regions 120. There is an additive effect for the two tensile stress regions with the two compressive stress regions, and the combined effect causes a greater voltage induced birefringence than can be obtained only with regions 119 or regions 120.

The TiC> 2 doped regions are loss-making for two reasons. The TiO2 “SiO2 glass tends to phase-separate and form small inhomogeneous scattering sites that increase scattering losses. TiC ^ also increases the refractive index of the region to a higher value than that of the region 117, so that the light emitted from the choir 116 reaching the regions 120 tends to break into and through the regions 120 and thus away from the choir 116. The -20 areas can be lost if they consist of glass bars that have been melted in crucibles containing such absorption impurities as iron, nickel, cobalt, copper, etc.

It is said in the above-mentioned GB patent no.

25,012,983, the methods described therein make it possible to produce fibers with voltage-induced birefringence Δη of up to 40 x 10, and that the so-called beat length L for such a value of Δη is 2.5 mm at a wavelength of 1 μι »and 1.25 mm at a wavelength of 0.5 µm. However, some applications require even shorter beat lengths, which require values of Δη of approx.

-3 10. The following theoretical example shows that forming values of Δη can easily be obtained in a fiber according to the invention. FIG. Fig. 13 shows a fiber having a chore 122, a lining 123 and two longitudinal regions 124 of circular cross-sectional shape. The choke 122 has a diameter of 5 µm, the voltage-producing 124 a diameter of 25 µm, and the casing 123 a diameter of 125 µm.

24

DK 15 5 4 7 4 B

The circular areas 124 are centered on a radius of 25 m. The specificity of the choke 122 is immaterial, as it is only necessary that its index of refraction be greater than the index of the cladding 123 consisting of pure SiC 2. The regions 124 consist of 5 wt% wt% and 83 wt% SiC 2.

The calculations of the double refraction are based on the publication: G. W. Scherer, "Stress-Induced Index Profile Distortion in Optical Waveguides", Applied Optics, Volume 10 19, No. 12, June 1980, pages 2000-2006. By means of a computer, the double-breaking caused by one of the areas 124 is calculated for the chore area and the inner lining, and then the double-breaking which the second region 124 causes in the central region was determined and added to the first calculated value. The results are plotted in FIG. 13. Lines 125, 126, 127 and 128 are lines where there are equal values of the double refraction of 0.4 x 10 10, 0.5 x 10,, 0.6 x 10 og, and 3003, respectively. 7 x 10, the latter line passing through the choir 20 122.

One method of providing the fiber of the invention uses a flame hydrolysis process of the same kind as disclosed in US Patents Nos. 3,737,292 and 4,165,223. The resulting fiber is shown in section 25 of FIG. 16, wherein elements of the same kind as in FIG. 11 have been given the same reference numerals with the fiber of FIG. 16 differs from the fiber of FIG. 11, the longitudinal, voltage-inducing regions 112 'have a crescent-shaped profile.

30 Referring now to FIG. 14. A layer of glass soot 130 is first deposited on a cylindrical glass mandrel 131 by means of a flame 133 emitted from a burner 132.

When the first soot layer 130 reaches a given thickness, the composition is changed and a second soot layer 35 134 is formed to form the inner lining 111 '. During application of the layers 130 and 134, the mandrel 131 is caused to rotate while the burner 132 is moved along the mandrel.

To form the soot to be consolidated into

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In order to remove the strips 112 ', the rotation of the mandrel is stopped while the burner 132 passes a sufficient number of times to form a soot layer 135. The mandrel 131 is rotated 180 ° and another soot layer 135 is provided on the diametrically opposed side, cf. 15. Similarly, layer 136 of garment sod can be deposited on layer 134 between strips 135. Thereafter, layer 137 of garment sod is applied, again causing the mandrel to rotate. The finished soot blank is porous and must therefore be heated to melt or "collapse" to obtain a monolithic glass blank from which an optical waveguide can be drawn as shown in FIG. 16th

The steps of applying the strips 136 of the glass can be omitted without affecting the geometric shape of the resultant fiber too much. If the coating layer 137 is directly applied to the outside of the portion of the soot blank comprising the layer 134 and the strips 135, the soot flow from the burner will deposit a larger amount of soot when the outside of the layer 134 faces the burner, 20 than when the outside of the strips 135 faces the burner. as the soot flow strikes the largest surface area when layer 134 faces the burner. This tends to make the cross-sectional shape of the blank more circular as layer 137 is formed. When a fiber is withdrawn from the consolidated blank, the surface tension tends to round the outer surface of the fiber, to a lesser extent affecting the circular shape of the choir. However, this does not have a major impact on the single-wave type fibers employed by the invention.

FIG. 17 shows an altered workpiece manufactured by the flame hydrolysis process, wherein elements of the same kind as in fig. 14 have been given the same reference numerals with *.

After providing the layers 130 'and 134' in the manner described with reference to FIG. 14, 35, a layer of sections 139 and 140 is provided as follows. The soot applicator must use a reactant feed system as described in U.S. Patent

4314837. This patent discloses a reactant additive.

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26, where reactant vapors are supplied to the flame hydrolysis burner through flow regulators, the flow rate being controlled by a control circuit. A shaft position indicator associated with the mandrel 131 'informs the control circuit of the system 5 about which portion of the soot surface is now facing the burner 132. A given reactant flow can be used to apply the regions 139, while applying to the regions 140 a doping reactant which affects the expansion coefficient of either the applied glass. Thus, while the mandrel 131 'rotates at a constant angular velocity, the regions 140 will be provided by the impulse-like supply of doping reactants to the burner. Due to mixing of the reactant vapors, there will be a transition area 15 between sections 139 and 140. On the layer comprising the regions 139 and 140, an outer layer 141 of coating material may be applied. After removal of the mandrel 131 ', in FIG. 17 is drawn to form fibers having in the cross-sectional plane a configuration of the same kind as that of FIG. 21, which will now be described in more detail below.

Instead of applying a soot-produced outer coating layer by the flame hydrolysis technique, this layer can be completely or partially eliminated and the outer coating is provided with a glass tube. After the 15 and 135 have been provided, or after the 16, including areas 111 'and 112', have been applied, e.g. remove the mandrel and consolidate the soda ash. The resulting dense glass blank 30 is inserted into a tube and the resulting combination is drawn into a fiber in accordance with the teachings of U.S. Patent No. 3,932,162.

FIG. Fig. 18 shows a rod-in-tube exit type that can be used to obtain fibers of the type shown in Figs. 11-35 13. A plurality of bars of suitable material are inserted into a tube 142 of cladding material such as SiC 2. Inside the tube 142 is placed a centrally located choke 143 of very clean glass and

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27 layers 144 of very pure glass with a lower refractive index than the choir 143. The choke 143 and the coating 144 are preferably provided by chemical vapor application technique.

In diametrical arrangement relative to the central bar 5, two bars 145 of high heat expansion coefficient glass are disposed relative to the tube 142. In orthogonal position relative to the bars 145, a second pair of bars 146. is provided. of the fiber of FIG. 11 and 13, the rods 146 10 may consist of the same material as the tube 142. To obtain a fiber of the type shown in FIG. 12, the rods 146 may consist of a material having a low coefficient of thermal expansion relative to the tube 142 and / or consisting of light absorbing material. Bars 147 of cladding material fulfill some of the spaces between the above-mentioned rods. If the resulting output blank comprises large unoccupied regions, those regions of the resulting fiber where tensile stress occurs will have a non-circular cross-sectional shape as shown in FIG. 11 20 due to the deformation of the rods as they fill the adjacent spaces. The core of such a fiber will also tend to be non-circular. If all of the above spaces are filled with thin bars of cladding material not shown in the drawing, the core and tension regions will have a more circular cross-sectional shape in the resulting fiber.

Pig. 19 shows a schematic view of an otherwise standard chemical vapor application apparatus modified for use in the practice of the invention. The apparatus comprises a substrate tube 150 which, on the downstream side, may be connected to a wider output tube 152. The tubes 150 and 152 are placed in a conventional, non-rotational age, which can be turned as indicated by the arrow.

A heater 156 which is movable in the directions indicated by arrows 158a 158a and 158b produces a hot zone 154 which can be moved through the tube 150. The heater body 156 may consist of any heat source, e.g. a plurality of burners circumscribing the tube 150.

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28 in the preferred embodiment, the heater must be capable of local heat application. For example, a single burner or two diametrically opposed burners may be used. The reactants are introduced into the pipe 150 through a pipeline 160 which is connected to a plurality of sources of gases and vapors. Any of the aforementioned reactant delivery systems may be used.

The burner 156 is first moved at low speed relative to the pipe 150 in the 10 direction indicated by arrow 158b corresponding to the direction of the reactant flow. The reactants react in the hot zone 154 and produce soot which is transported by the gas stream while a portion of this soot is deposited on the inside of the tube 150 in the region 162. While the burner 156 continues to move in the direction indicated by arrow 158b, the heat moves zone 154 is in the direction of flow so that the soda accumulation expands to the hot zone and consolidates to form a monolithic, homogeneous glassy layer on the inside of the tube 150.

As the burner 156 reaches the end of the tube 150 at the exit tube 152, the flame temperature is reduced and the burner is returned in the direction indicated by arrow 158a to the input end of the tube 150. Thereafter, additional layers of glass material within the tube 25 150 are applied to the above described manner.

After applying suitable layers to form the core material and any other desired layers to form the optical fiber, the temperature of the glass is increased to collapse the tube 150. This can be achieved by decreasing the speed at which the hot zone moves. Preferably, during this operation, an appropriate pressure is established in the interior of the tube 150, cf.

U.S. Patent No. 4,154,591.

The conventional apparatus described above is well suited for applying glass layers of uniform composition to the inside of the tube 150. In accordance with the invention, the conventional apparatus is modified by the proximity and upstream side of the hot zone.

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29 154 is provided with means for supplying two diametrically opposed regions of the hot zone of reactant gases capable of providing soot having an expansion coefficient different from the coefficient 5 of the lining material. As can be seen in Fig. 19, a portion of two gas supply tubes 164 extends to the end of the substrate tube 150 into which the reactants are introduced. These portions of the tubes 164 within the tube 150 terminate immediately before the hot zone 154. The tubes 164 10, as indicated by a dashed line 166, have a mechanical connection with the burner 156 so that the tubes 164 are kept at a suitable distance on the upstream side of the hot zone 154 Alternatively, the heat source and tubes 164 are held stationary while tube 150 is axially moved. The input end of the tube 150 is connected to the tubes 164 by means of a bellows 168, while a gasket 170 connects the bellows 168 to the tube 150. When not in use, the tubes 164 can be completely pulled away from the tube 150 or partially removed from the tube. , ie so much so that they do not disturb the flow of the reactants from the conduit 160 to the conduit 150.

Referring now to FIG. 20. A coating 178 of the glass can be applied to the inside of the tube 150 in a conventional manner. To form diametrically opposed, 25 longitudinal expansion strips within the casing, another reactant material is fed through the tubes 164, while the reactant material for the casing continues to flow through the tube 160. For example, the tube 160 can be fed with SiCl 2 and BCl layer 178 of garment glass. After the layer 178 is of sufficient thickness, the tubes 164 are positioned near the hot zone and fed with reactant material such as GeCl 2, while the tube 160 continues to be fed with SiCl 2 and BClg. Oxygen to be used for reaction is also supplied to the hot zone in a manner known per se.

A layer 180 of borosilicate glass is deposited on the inside of the layer 178, and the dotted portions 182 of the layer 180 contain GeO2.

Claims (14)

10 After providing the FIG. 20, the blank is caused to collapse to form a solid blank which is then heated in an oven to a sufficiently high temperature for a fiber to be drawn. The resulting fiber shown in FIG. 21 is shown in cross-section, comprising a choir 190, an inner cladding area 192 and an outer cladding area 196. On opposite sides of the choir 190 and within the area 196 there are two longitudinal regions 194 consisting of high expansion coefficient glass. There is a gradual change between the regions 194 and the surrounding glass due to mixing of gases during application of the glass and due to the diffusion of the dopant materials during the various stages of exposure of the glass to high temperature. The fiber of the configuration shown in cross-section 25 of FIG. 21, will also result from the consolidation and drawing of a soot blank of the type shown in FIG. 17th A single-wave optical polarization-preserving fiber of the type comprising a transparent core (110, 116, 122, 190) surrounded by a layer (113, 117, 118, 123, 196) of transparent coating material having refractive index lower than the refractive index of the choir, and wherein the cladding layer has an asymmetry that produces double refraction in the choir, characterized in that the cladding layer comprises a pair of diametrically opposed longitudinal glass regions (112, 119, 124, 194) whose coefficient of expansion is different from the coefficient of thermal expansion of the casing. Optical waveguide according to claim 1, characterized in that the coefficient of thermal expansion of said pair of diametrically opposed regions (112, 119, 124, 194) is greater than the coefficient of thermal expansion of the coating glass (113, 117, 118, 123, 196). 3. Optical waveguide according to claim 1, characterized in that the heat expansion coefficient of said pair of diametrically opposed regions (112, 119, 124, 194) is less than the heat expansion coefficient of the glass (113, 117, 118, 123, 196). . Optical waveguide according to claim 1, characterized in that the fiber further comprises a second pair of longitudinal regions (120) in orthogonal arrangement relative to said two diametrically opposed regions (119), which second pair of regions (120) has other physical properties than said first few areas. Wave conductor according to claim 4, characterized in that the coefficient of thermal expansion of said second pair of regions (120) is less than the coefficient of thermal expansion of the coating glass (117, .118). A single-wave optical polarization-preserving fiber (70) of the type comprising a transparent core (80) surrounded by a layer (74, 78, 82) of transparent coating material having a refractive index lower than the refractive index of the core, and wherein the coating material forms a exterior lining (74) of cladding lenses surrounding an inner liner and having a coefficient of thermal expansion different from the liner coefficient of thermal expansion, the outer liner (74) having a generally circular cross-sectional shape and having the liner 35 asymmetry which causes double breaking in the choir, DK 155474 B, characterized in that the choir (80) has an elongated cross-sectional shape, that the cladding layer (74, 78, 82) is an inner elongated cladding layer (78, 82) on the surface of the choir (80). ), and said inner liner comprises a very clean glass optical liner (78) surrounded by a layer (82) of lower purity glass. Fiber according to claim 6, characterized in that the ratio of the cross-sectional area of the outer covering layer (74) to the cross-sectional area of the fiber is greater than 0.9. Fiber according to claim 6 or 7, characterized in that the thickness of the optical covering layer (78) is greater than twenty times the working wavelength of the fiber. Fiber according to any one of claims 6-8, characterized in that the difference between the coefficient of thermal expansion of the cladding glass and said inner cladding glass is greater than 1-10 V ° C. Fiber according to claim 9, characterized in that the coefficient of thermal expansion of the inner casing glass is greater than the coefficient of thermal expansion of the stress coating glass. A method of producing an optical fiber output blank for use in the manufacture of an optical polarization preservative fiber according to any one of the preceding claims, wherein a glass tube (142) is provided which is placed a glass rod centrally in said tube ( 142), a pair of glass bars (145) are placed on diametrically opposed sides of the first-mentioned glass rod, and several glass bars (147) are placed at least in some of the spaces between the central rod, the two diametrically opposed bars, and the tube, characterized in that as a central rod in the glass tube (142) a rod consisting of an axially located choir (143) surrounded by a layer of cover glass (144) is used and that for said opposite glass bars is used bars (145) of glass having a coefficient of expansion of heat different from the coefficient of thermal expansion of the cladding glass (144). Method according to claim 11, characterized in that in the tube (142), on opposite sides of the central glass rod and orthogonally with respect to the first pair of glass bars (145), a second pair is preferably light absorbing glass bars (146). ) having a coefficient of thermal expansion different from the coefficient of thermal expansion of the material of the first pair of glass bars (145). A method according to claim 12, characterized in that the material of said second pair of glass bars (146) is selected such that its coefficient of thermal expansion in relation to the thermal expansion coefficient of the cladding glass (144) differs in a direction different from the direction in which the former the heat expansion coefficient of pair of glass bars 20 (145) differs from the heat expansion coefficient of the cladding glass layer (144). Method according to any one of claims 11 to 13, characterized in that as a central rod with axially located cores (143) and surrounding cladding glass layer (144), a rod is used in which the outer radius of the cladding glass layer is at least five times the cores ( 143) outer radius.